The present invention relates to gas turbines and, more particularly, to industrial or heavy-duty gas turbines. Even more particularly, the present invention relates to modified buckets in the first turbine stage of a gas turbine engine for reducing the probability of radial cracking.
The efficiency of thermal engines is improved by increasing the temperature of the heated fluid being employed. In a gas turbine engine, the heated fluid is a mixture of air and combustion products produced by burning fuel. This heated gas mixture is impinged on buckets of one or more turbine stages to produce torque. The maximum temperature which can be used is limited by the availability of materials which can withstand deformation and/or destruction at a given temperature. To maximize efficiency in a modern industrial heavy-duty type turbine, the turbine buckets are produced from special alloys which exhibit high strength and toughness retention at elevated temperatures. Such alloys and the processes for casting and finishing the turbine buckets are expensive. Furthermore, the cost of a gas turbine engine is great enough that a long useful life must be anticipated for economical use.
In order to reduce the rotating mass and radial forces on the dovetail region and rim of a turbine wheel, and to improve tip sealing, it has been customary to core or hollow an outer portion of the buckets especially of the first-stage turbine of a gas turbine engine. For greatest reduction in weight, the remaining walls of the cored portions should be as thin as possible. The wall thinness is limited in the region of the trailing edge which customarily is thinned down almost to a knife edge. Consequently, it has been conventional to leave an uncored section along the trailing edge behind the cored portion.
The thin walls appear to be subject to vibratory excitation which may produce stress concentrations at the junction of the uncored trailing edge with the walls. At least two types of vibratory excitation appears to be capable of superposing contributions to stress concentrations at this junction, particularly at the tip. A third source of stress concentrations, namely grooves or striations from tip rubbing, can also occur in this same location to produce an enhanced opportunity for crack initiation.
One solution which has been applied is to increase the thickness of the walls of the cored portion to thereby raise the resonant vibratory frequencies. Although this may be effective in reducing radial cracking, it is contrary to the desire for reduced weight and loading of the buckets.
Once radial cracks have begun, they may propagate to destructive failure thus seriously damaging or destroying expensive apparatus. When cracks are discovered, there are few alternatives to replacement of the affected bucket. If cracks are discovered when very small, there is the possibility that they can be ground out with a consequent reduction in aerodynamic efficiency of the turbine stage and with an imbalance which must be cured possibly by correspondingly grinding an opposed bucket. Since turbine buckets are produced from high-cost superalloys, the cost of replacement is substantial.